The present invention relates to the drying section of a paper making machine and particularly to the steam pressure profile of the drying cylinders of the drying section.
In a paper making machine, a web of paper is formed as a very wet pulp suspension, is drained of some water in a forming section, is pressed partially dry in a press section and is delivered to pass through a drying section. Paper making machine drying sections typically include a series of hollow, metal drying cylinders or dryers, which are fed pressurized steam to heat the peripheral surfaces of the cylinders. The web to be dried is led along a path that brings it into contact with the peripheral surface of each of the drying cylinders in sequence. The heat transferred to the web by each drying cylinder heats the web and enhances the evaporation of water from the wet web, ultimately bringing the web to a high percentage of dryness. For example, a web may enter the drying section as wet as only 45% dry and may leave the drying section as much as 92% to 98% dry. The amount of heat supplied to the paper web as it moves past each drying cylinder is a function of the temperature of the peripheral surface of the cylinder over which the web passes. The temperature is in turn a function of the pressure of the steam in the cylinder. A cylinder with higher pressure steam is hotter.
All of the drying cylinders in a drying section are fed with steam which may be from a common steam source. The pressures in different cylinders may not all be maintained at the same level, which means also that the cylinder temperatures vary. The steam pressure may be controlled by the manner in which steam is fed to the cylinders or by valving of the steam to the cylinders. As the web becomes progressively drier, different cylinder temperatures provide optimum drying of the web.
The invention is applicable to a particular configuration of a drying section. A drying section comprises a series of drying cylinders which are typically operated in a plurality of separate groups of the drying cylinders. The drying groups typically are either of the double tier or single tier variety.
In a double tier dryer group, the active heated drying cylinders are arrayed in two rows. The web is passed through the double tier group by one side of the web first contacting a drying cylinder in one row, and the other side of the web next contacting a drying cylinder in the other row, and one side of the web next contacting a drying cylinder in the first row, then the other side of the web contacting a drying cylinder in the second row, and so forth through the entire double tier drying group. In each case, the web wraps around drying cylinders, one after the other.
In each single tier drying group, in contrast, only one side of the web, e.g. the underside of the web, is in contact with each of the drying cylinders in the group. In order to desirably increase the angle of wrap of the web around each drying cylinder in the single tier drying group, there is a web guiding and reversing roll, typically in the form of a vacuum roll, between adjacent drying cylinders in the dryer group. That reversing roll is placed to cause the web to wrap around a significant portion of the circumference of the adjacent drying cylinders in the single tier group.
A web supporting, dryer felt belt or felt is provided for each single tier drying group. The felt supports one surface of the web and carries the web from drying cylinder to drying cylinder within the single tier dryer group. In a typical double tier drying group, the path of the web from a drying cylinder in one row to a drying cylinder in the other row is not felt supported.
Other known drying cylinder arrangements within a drying group are known.
A design that is a hybrid of at least one single tier drying group feeding the partially dried web to at least one double tier drying group was known in the early 1980's. Initially, such designs typically included a small quantity of the drying cylinders in the single tier group(s), followed by a significantly larger quantity of the drying cylinders in the double tier arrangement of one or more double tier groups. The few drying cylinders in the single tier groups mostly did not dry a web but rather heated the web for enabling the actual drying that took place in the double tier groups. As the hybrid arrangement evolved into the early 1990's, a larger portion of the drying cylinders were placed in the single tier groups and a smaller portion of the drying cylinders were placed in the double tier groups. A significant part of the drying, therefore, took place in the single tier drying groups, while the final drying took place in the double tier drying groups.
In some paper making machines, the dried web is subject to subsequent treatment, e.g. application of sizing in a size press or even coating on line after the drying section. The sizing applied is too sensitive to a high temperature dried web. Efforts have been made in the final part of the drying section, particularly in the double tier dryer groups of the hybrid arrangement, to control the temperature of the web at the end of the drying section for optimum web production. The present invention concerns a drying section heat control arrangement obtained through control of the delivery of steam pressure to at least certain drying cylinders.